Leonard C Suskin's musings on writing, parenthood, and the wonderful world of commercial AV.

Tuesday, April 3, 2012

C is for Contrast Ratio. Also Caculation

It's a system installer's nightmare-scenario, and one that happened to me. The installation is just finished, the last wire connected, the control system merilly and efficiently controlling everything with a convenient and efficient push of a button. You see what you're supposed to see, hear what you're supposed to hear, bring the client in and... his face falls. "That's what it looks like? I expected it to be ... brighter". And, yes, when you look at the shiny new projection screen you just installed the image looks washed out. Very washed out. How did you get here? And how could you fix it? The answer in this case is math.

In designing an AV system there are a dizzying array of options as to how these goals will be accomplished. Video, for example, can be delivered via an LCD of flat-panel display, front- or rear-projection system, or a video-wall made of several smaller displays. Audio could be delivered via speakers mounted on a ceiling or wall, in a multi-channel (stereo or surround-sound format), single zone, mix-minus, or some combination. Sometimes follows function (a large paging system would likely use relatively low-cost cnstant-voltage speakers, while a large performance venue might use line-arrays to reduce the attenuation due to distance), some choices will be dictated by cost, and some by architectural and aesthetic constraints. Whatever the choices made, however, it should be possible to justify them mathematicaly; AV design can be an art, but at its core it is also a science. This example involved a simple front-projection presentation system and a real-world demonstration of the importance of contrast ratio.

One of the questions one has to ask is how bright a projector is needed. We can start answering that question by looking at contrast ratioA year ago Infocom (the AV trade organization for those of you who found me more through ink than pixels) released a new standard for what is considered an acceptable contrast ratio in a video projection system. That standard calls for ratios of:

8:1 for general viewing
10:1 for reading
12:1 for detail viewing
80:1 for cinema quality video.

So, for an average meeting room in which you're projecting text, full white shoult be just ten times the brightness of full black. If you know the ambient light incident on the screen it's easy to see how big a projector you'd need:

A glance at the formula will tell you how you can get such a high ratio for cinema; movie theaters are very dark. Other kinds of rooms can't be as dark because people like to see what they are doing (another constraint is that "task lighting" - shoudn't be more than three times ambient light, to allow your eyes to adjust. Contrast can also, of course, be measured easilly enough with a light meter and a "checkerboard" test pattern of alternating white and black squares. In this particular case the room had already been built, te projector installed - and now we saw that it looked washed out. A quick scan with the light meter confirmed what was apparent at a glance; the room had floor-to-ceiling south-facing windows and was, therefore, as bright as daylight. This left the simple solution of getting some darker shades with a definite and very reachable target for illumination incident to the screen.

Was that obvious from the first? Of course. But the math let's you quantify - to answer how much darker must the room be, or how much brighter the projector.

There is an art to AV system design, to understanding the users' needs and finding the neatest, most elegant way to meet them. There is also a science, and numbers to tell you exactly what you need and what you should expect.

So, that was 'C'. A bit of a change of pace, no?

I'll be back tomorrow with the letter D for.. digital publishing? Displayport? DRM? Tune in tomorrow to find out!